1. Field of the Invention
The present invention relates to a battery pack including cylindrical lithium ion secondary batteries, and a rechargeable vacuum cleaner including the battery pack.
2. Description of the Related Art
Lithium ion secondary batteries are small and lightweight, and can be charged and discharged more than 300 cycles repeatedly, and are hence used widely in laptop computers, cellular phones and many other electronic appliances, and the demand is increasing. This is because the weight energy density of the lithium ion secondary battery (LIB) is higher than in the nickel-cadmium secondary battery (Ni—Cd) and nickel mercury secondary battery (Ni-MH). The weight energy density is 44 Wh/kg in Ni—Cd, 54 Wh/kg in Ni-MH, and 150 Wh/kg in LIB, and it is outstandingly high in the LIB.
In various electronic devices such as a digital camera, consumer electric appliances such as a video camera and a cordless cleaner, an electric-assisted bicycle, and the like, so far, Ni—Cd batteries have been used, but in the recent trend toward smaller size and lighter weight, the power supply unit is being converted from the Ni—Cd to the Ni-MH, and further to the LIB. Although the demand for the LIB is increasing, it is not replacing the Ni—Cd or Ni-MH completely.
At the present, the LIB is used in products of a relatively small current consumption. When expressed in C rate (the current discharging the full charged capacity of a unit cell in one hour), it is about 1C, which is a relatively small current of about 1 A. Applicable products are a personal computer, a cellular phone, a digital camera, and a portable audio device (walkman), and it is not applied in products of large current consumption, such as a power tool for driving a motor, a cordless cleaner, an uninterrupted power source (UPS), an electric-assisted bicycle, and a hybrid type car. The reason is that dendrite of metal is likely to precipitate on the negative electrode of the LIB when charging and discharging of large current are repeated, and precipitates of lithium dendrite may reach up to the positive electrode through the separator, thereby causing internal short-circuit, which leads to a safety problem.
Not limited to cylindrical or prismatic type, charging and discharging reaction of the lithium ion secondary battery hardly occurs uniformly on the entire surface of the electrode, but tends to be concentrated in the central area. Accordingly, when charged or discharged at high rate, dendrite of Li metal is likely to grow in a short time, especially on the negative electrode. In the Ni-MH secondary battery, the internal resistance is extremely low, being about several micro-ohms, and Joule heat generation is very small, and hence safety can be assured even if discharged continuously at high rate. In the lithium ion secondary battery, on the other hand, the internal impedance is 50 to 80 mΩ, being nearly 20 cycles that of the Ni-MH. Therefore, if the lithium ion secondary battery is discharged at high rate in the same condition as in the Ni-MH secondary battery, the temperature inside of the battery is raised by the Joule heat, and dendrite growth is promoted, and the incidence rate of internal short-circuit increases. Owing to this reason, the lithium ion secondary battery is not used in high rate discharge applications, and hence is cannot be applied in products consuming a large current such as a cordless cleaner, a robot, a UPS, and an electric-assisted bicycle.
Meanwhile, Jpn. Pat. Appln. KOKAI Publication No. 10-270095 discloses, in claim 1, a cooling device for battery power source, being a device for cooling a multiplicity of battery modules in a holder case, having a multiplicity of battery modules formed by connecting a plurality of unit cells in a row and connected in series electrically and mechanically, in the holder case, and moving the air by force in the holder case in one direction, in which the flowing direction of the air is a direction orthogonal to the longitudinal direction of the battery module.
In this cooling device, as mentioned in paragraphs [0009] and [0024] in the publication, unit cells composing each battery module are cooled uniformly. Hence, when this cooling device is used for cooling the lithium ion secondary battery, long life is not expected in the charging and discharging cycles for discharging at high rate.